One of the design challenges in mobile communication receivers, such as personal communication terminals, PCTs, and mobile cellular phones, is dealing with signal fading due to signals propagating to a receiver through various paths. These paths are referred to as multipaths leading to what is known as multipath propagation.
One of the problems with multipath propagation in wireless communication systems is the fading caused by reflection and scattering of radio signals from buildings, trees and other obstacles along the radio path. Radio waves or signals arrive at a mobile receiver from many different directions, with different time delays or phase lags. Direct signal rays, ground signal rays and other possible scattered rays combine vectorially at the receiver antenna to give a resultant signal which depends on the differences in path length that exist in the multipath field.
The amplitudes and phases of signals received from a transmitter through different multipaths of a channel are generally independent of each other. Because of complex addition of multipath signals, the strength of received signals may vary between very small and moderately large values. Thus fading is commonly referred to as the phenomenon of received signal strength variation due to complex addition of multipath signals. In a fading environment, points of very low signal strength, or deep fades, are separated by approximately one-half of a signal wavelength from each other. Typically, multipath fading in a wireless communications environment can create 20-30 dB deep fades and deteriorate signal-to-noise (SNR) ratio at the cellular handsets, which causes poor voice quality.
One approach to overcome the fading problem is disclosed in a United States pending application, Ser. No. 08/741,999, (Chen 1-1-4), filed on Oct. 14, 1996 and assigned to the same assignee as the current application, which is incorporated herein by reference. Basically, the prior system employs in a wireless receiver a first and second antenna, with each antenna receiving a corresponding signal. Typically, the second antenna is located one quarter to one half phase away from the first antenna. A phase shifter is configured to shift the phase of the signal received by the second antenna in RF (Radio Frequency) stage. As well, means for combining the first and second signals in the RF stage is provided. Power detection means for detecting the combined power for the first and second signals is utilized. An adaptive controller means controls the phase of the phase shifter so that the phase shifter shifts the phase of the second signal such that the power of the first and second signals are constructively combined.
However, in certain circumstances, the constructive combination of the two signals from each one of the antennas may lead to the saturation of amplifiers employed in the front end stage of the receiver. For example, when the received signals are in-phase, their constructive combination results in a signal having an amplitude twice as large as that of only one of the received signals. Prior art systems require a specifically designed attenuator so that the power level of the received combined signals do not exceed a given threshold above which results in amplifier saturation.
Furthermore, in a rapidly signal-changing environment, there is a need to track signal variation at a substantially rapid speed, whereas, in a slowly signal-changing environment, it is desirable to maintain a substantially accurate tracking. Prior art systems do not teach or provide a feedback control arrangement that allows for such adaptive tracking of fading signals.
Thus, there is a need for an adaptive controller that is capable of efficiently tracking rapidly varying signal conditions without sacrificing accuracy. It is also preferable to maintain the power level of the received combination signal below a given threshold without the need for a specifically designed attenuator.